As the world population continues to increase and as industries and cities continue to expand, the demand for various natural resources gives rise to a demand for improved techniques. Thus, while for some purposes, water can be used without purification, pollution and contamination from various sources renders most of the natural water supplies unsuitable for drinking supplies for humans and domestic animals. Similarly, many lakes and rivers can no longer be used safely for swimming because of pollution. Hence it has become necessary in recent years to construct more and more swimming pools for private and public use. Hence, there is an increasing need for a cheap, simple and reliable method for purifying water so that it can be safely used for such purposes.
While many municipalities pass their water-supplies through a treatment plant in which the water is purified by various methods including chlorination such plants are expensive and are not completely without disadvantages. Hence, it is often found that when water used for drinking or in swimming pools is subjected to a conventional chlorine treatment, the water source is often so highly polluted that excessive chlorination by chlorine is required. In such cases, over-chlorination is sometimes neutralised by the addition of a scavenging agent, such as sulphur dioxide. Water intended for swimming pools is usually treated with chlorine or other halogens directly and often contains a great excess of chlorine. This excess of chlorine is very irritating to the eyes of the swimmers.